Since the discovery of oxide superconducting materials with transition temperatures above about 20 Kelvin the possibility of using them to obtain greater efficiency in electrical and magnetic applications has attracted considerable interest. However, to be practical outside the laboratory, most electrical and magnetic applications require flexible cabled lengths of conductor manufacturable with high packing factors which can be manufactured at reasonable cost and with high engineering current-carrying capacity. High packing factor forms are needed because limited space constraints and high overall current requirements are major design issues. Conductors which are flexibly cabled, that is, composed of twisted, helically wound, braided or otherwise transposed bundles of electrically, and sometimes mechanically, isolated conductor strands, are desired in many applications, including coils, rotating machinery and long length cables. In comparison to monolithic conductors of comparable composition and cross-section, cabled forms which are made from a number of isolated conductors strands will have much higher flexibility. Substantially mechanically isolated cable strands have some ability to move within the cable, although some degree of mechanical locking of the strands is desired for stability and robustness of the conductor to stay together during handling and winding. Electrical isolation of the cable strands is preferred but not required. In low temperature superconducting conductors, cables which are made from a number of substantially electrically isolated and transposed conductor strands have been shown to have greatly reduced AC losses in comparison to monolithic conductors. See "Superconducting Magnets" by Martin Wilson (1983,1990), pp 197, 307-309. It has been proposed that the same relation will hold for high temperature superconductors. Flexibility increases in proportion to the ratio between the cable cross-section and the strand cross-section. AC losses are believed to decrease in relation to cable cross-section, strand cross-section and twist pitch. Thus, the greater the number of strands in a cable of given dimension, the more pronounced these advantages will be.
However, it has not been considered feasible to form oxide superconductors in high winding density, tightly transposed configurations because of the physical limitations of the material. Superconducting oxides have complex, brittle, ceramic-like structures which cannot by themselves be drawn into wires or similar forms using conventional metal-processing methods and which do not possess the necessary mechanical properties to withstand cabling in continuous long lengths. Consequently, the more useful forms of high temperature superconducting conductors usually are composite structures in which the superconducting oxides are supported by a matrix material, typically a noble metal, which adds mechanical robustness to the composite.
Even in composite forms, the geometries in which high-performance superconducting oxide articles may be successfully fabricated are constrained by the relative brittleness of the composite, by the electrical anisotropy characteristic of the oxide superconductor, and by the necessity of "texturing" the oxide material to achieve adequate critical current density. Unlike other known conductors, the current-carrying capacity of a superconducting oxide composite depends significantly on the degree of crystallographic alignment and intergrain bonding of the oxide grains, together known as "texturing", induced during the composite manufacturing operation.
Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating anneal and deformation steps. The anneal is used to promote reaction-induced texture (RIT) of the oxide superconductor in which the anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of the oxide grains (deformation-induced texturing or DIT).
The texture derived from a particular deformation technique will depend on how closely the applied strain vectors correspond to the slip planes in the superconducting oxide. Thus, superconducting oxides such as the BSCCO family, which have a micaceous structure characterized by highly anisotropic preferred cleavage planes and slip systems, possess a highly anisotropic current-carrying capacity. Such superconducting oxides are known to be most effectively DIT textured by non-axisymmetric techniques such as pressing and rolling, which create highly aspected (greater than about 5:1) forms. Other methods of texturing BSCCO 2223 have been described in U.S. Ser. No. 08/302,601, filed Sep. 8, 1994 entitled "Torsional Texturing of Superconducting Oxide Composite Articles", which describes a torsional texturing technique; U.S. Ser. No. 08/041,822 filed Apr. 1, 1993, entitled "Improved Processing for Oxide Superconductors" now issued as U.S. Pat. No. 5,635,456; and U.S. Ser. No. 08/198,912 filed Feb. 17, 1994, entitled "Improved Processing of Oxide Superconductors" which is now issued as U.S. Pat. No. 5,635,456 which describes an RIT technique based on partial melting. These techniques have been observed to provide the greatest improvement in the Jc's of BSCCO 2223 samples when used in combination with a highly non-axisymmetric DIT technique, such as rolling.
Although superconducting oxide composite articles may be textured by various methods, including magnetic alignment, longitudinal deformation (DIT) or heat treatment (RIT), not all texturing methods are equally applicable to, or effective for, all superconducting oxides. For example, known techniques for texturing the two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors (Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x, also known as BSCCO 2212 and BSCCO 2223, respectively) are described in Tenbrink et al., "Development of Technical High-T.sub.c Superconductor Wires and Tapes", Paper MF-1, Applied Superconductivity Conference, Chicago(Aug. 23-28, 1992), H. B. Liu and J. B. Vander Sande, submitted to Physica C, (1995), and Motowidlo et al., "Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors", paper presented at Materials research Society Meeting, Apr. 12-15, 1993. Micaceous oxides such as the BSCCO family which demonstrate high current carrying capacity in the absence of biaxial texture have been considered especially promising for electrical applications because they can be textured by techniques which are readily scalable to long-length manufacturing.
Liquid phases in co-existence with solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of the phase diagram containing the oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperature is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x, (BSCCO 2212, where 0.ltoreq.x.ltoreq.1.5), into an alkaline earth oxide and a liquid phase and of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO 123, where 0.ltoreq..delta..ltoreq.1.0) into Y.sub.2 BaCuO.sub.5 and a liquid phase are well known. Kase et al. in IEEE Trans. Mag. 27(2), 1254 (1991) report obtaining highly textured BSCCO 2212 by slowly cooling through the peritectic point, a RIT technique because BSCCO 2212 totally melts and reforms during melt textured growth, any texturing induced by deformation prior to the melting will not influence the final structure.
However, BSCCO 2223 cannot be effectively textured by the melt-textured growth technique. Instead of peritectic melting, BSCCO 2223 exhibits irreversible melting in that solid 2223 does not form directly from a liquid of 2223 composition. RIT techniques applicable to BSCCO 2223 rely on some type of partial melting, such as eutectic melting, solid solution melting or formation of non-equilibrium liquids, in which the oxide superconductor coexists with a liquid phase rather than being totally decomposed.
Partial melting of (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x, ((Bi,Pb)SCCO 2223, where 0.ltoreq.x.ltoreq.1.5) and (Bi).sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.10+x ((Bi)SCCO 2223, where 0.ltoreq.x.ltoreq.1.5) at temperatures above 870.degree. C. in air has been reported; see, for example, Kobayashi et al. Jap. J. Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov. 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap. J. Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.
Such partial melting techniques are inherently more dependent on the geometry of the initial phase than melt-textured growth, and texturing induced by deformation prior to the partial melting will have a significant impact on the texturing of the final product. In short, for superconducting oxides with irreversible melting characteristics, such as BSCCO 2223, superior texturing and current-carrying capacity are most obtainable in highly aspected forms such as tapes.
Unfortunately, highly aspected superconducting oxide tapes are particularly difficult to cable. All superconducting oxide composites are brittle by the standards of conventional conductors. It is well known that exerting any bend strain in excess of a critical strain which is determined by the composition and geometry of the composite (and which is typically on the order of 0.1-1%) will severely degrade its electrical and mechanical properties. Strands which are round in cross-section can be bent in any plane and the bend strain will be the same, but the strain on a highly aspected strand will depend on the bend direction, with highest strains when the bend is in the plane of the longer cross-sectional dimension. The effect on strand performance can be considerable, since the bend strain increases proportionally to the thickness of the bent material and the critical current drops asymptotically at bend strains in excess of the critical strain.
Since the lowest coupling losses are predicted to come from fully transposed cables, limitations on the direction in which the strands can be cabled also limits the potential usefulness of the cabled conductor. Unfortunately, some forms of transposition makes it inevitable that some portion of the cabled conductor will not be oriented in the preferred direction. Thus, an important consideration in fabricating high performance oxide superconducting conductors is maximizing the portions which do have the desired orientations. It is thus desirable to main a common orientation for all strands in the cable. In rigid cabling techniques the oxide superconducting strands rotate around cable axis resulting in strands of various orientations. In planetary cabling, the oxide superconductor strands do not rotate and transposition only results in slight misorientation.
The difficulties of handling superconducting oxide strands appear even more pronounced when the need for a low cost, scalable cable manufacturing process is considered. There are a number of well-known cabling techniques, such as Rutherford cabling, braiding, and other forms of Litz cabling, for transposing low aspect ratio strands of conventional conductor material on automated machinery, which rely on gradual radial bending of the conductor strands, but to make high packing factor cables on these machines requires bending strains in excess of those tolerated by conventional oxide superconductor strands. The problem is even worse for aspected forms. The best-known automatic technique for cabling conventional highly aspected conductors requires sharp bends in the strand at regular intervals and so, not surprisingly, has never been demonstrated to be practicable for oxide superconducting composites.